This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Television broadcast systems throughout the world have migrated from the delivery of analog audio and video signals to modern digital communications systems. For example, in the United States, the Advanced Television Standards Committee (ATSC) has developed a standard called “ATSC Standard: Digital Television Standard A/53” (the A53 standard). The A53 standard defines how data for digital television broadcasts should be encoded and decoded. In addition, the U.S. Federal Communications Commission (FCC) has allocated portions of the electromagnetic spectrum for television broadcasts. The FCC assigns a contiguous 6 MHz channel within the allocated portion to a broadcaster for transmission of terrestrial (i.e., not cable or satellite) digital television broadcasts. Each 6 MHz channel has a channel capacity of approximately 19 Mb/second based on the encoding and modulation format in the A53 standard. Furthermore, the FCC has mandated that transmissions of terrestrial digital television data through the 6 MHz channel must comply with the A53 standard.
The A53 standard defines how source data (e.g., digital audio and video data) should be processed and modulated into a signal that is transmitted through the channel. The processing adds redundant information to the source data so that a receiver that receives the signal from the channel may recover the source data even if the channel adds noise and multi-path interference to the transmitted signal. The redundant information added to the source data reduces the effective data rate at which the source data is transmitted but increases the potential for successful recovery of the source data from the transmitted signal.
FIG. 1 shows a block diagram of a typical transmitting system 100 that transmits signal compliant with the A53 standard. Data is generated by a transmission source 102 and is arranged into packets. The packets are 187 bytes in size and may contain one or more codewords. Each packet includes a 3-byte header of which 13-bits are a packet ID (PID) that identifies the type of data that are sent in the packet. For example, a packet with a PID that has a value of 0x11 (hex 11) may identify the content as having a first video stream and a packet comprising a PID that has a value of 0x14 may identify the contents of such packet as a first audio stream. A data randomizer 104 randomizes the packet and provides the packet to Reed-Solomon encoder 106. The Reed-Solomon encoder 106 calculates and concatenates 20 parity bytes to the randomized data to produce a R-S packet that has 207 bytes.
A convolutional interleaver 108 interleaves the R-S packet in order to further randomize the data in time. A trellis encoder 110 encodes the interleaved packet to produce a block of 828 3-bit symbols. The A53 standard specifies the use of 12 trellis encoders, wherein each trellis encoder is a ⅔-rate trellis encoder producing a 3 bit symbol for every two bits present in the interleaved packet. As a result, the trellis encoder 110 includes a de-multiplexer, twelve parallel ⅔-rate trellis encoders, and a multiplexer. Data from the convolutional interleaver 108 is de-multiplexed and distributed to the twelve trellis encoders and the symbols generated by the twelve trellis encoders are multiplexed into stream of symbols.
A sync multiplexer 112 inserts 4 predefined segment sync symbols at the beginning of each 828-symbol block to create an 832-symbol segment. In addition, the sync multiplexer 112 inserts a field sync comprising 832 symbols for every 312 segments that are generated. In particular, the field sync symbols precede the 312 segments.
An 8-VSB modulator 114 uses the multiplexed symbols, including the data encoded by the trellis encoder 110, the segment sync symbols, and the field sync to modulate a carrier signal using 8-VSB (vestigial sideband) modulation. Specifically, the 8-VSB modulator 114 generates a signal, wherein the amplitude of the signal is at one of 8 discrete levels, where each discrete level corresponds to a particular 3-bit symbol. The signal is thereafter converted from digital to analog signal format and up-converted to radio frequency, using circuitry not shown. The radio frequency signal is transmitted using an antenna 116. Typically, the combination of the data randomizer 104, the Reed-Solomon encoder 106, the convolutional interleaver 108, and the trellis encoder 110 are referred to as an 8-VSB encoder 120. 8-VSB encoder 120 may also be referred to as an A53 encoder or ATSC encoder.
The data generated by the transmission source 102 includes video that is source encoded using the motion picture entertainment group (MPEG) 2 format that is also equivalent to International Standards Organization/International Electrotechnical Commission (ISO/IEC) 13818-2 format. The transmission source 102 also includes audio data that is source encoded using Dolby Arc Consistency algorithm #3 (AC-3). The A53 standard also allows the use of metadata for other program elements such as program guide data and such program elements may be source encoded using other methods. In addition, The A53 standard allows transmission of video at a variety of video quality levels and display formats ranging from standard definition interlaced television quality to progressive scan widescreen high definition quality. The FCC requires that broadcasters must use the A53 standard to encode data generated by the transmission source 102. If the transmission of a digital television program broadcast does not require the entire 19 Mb/second capacity of the allocated channel, the broadcaster may use any excess capacity to broadcast other services, possibly even to devices such as portable receivers and cellular telephones. However, the FCC requires that any data transmitted to such other devices using the excess capacity must be transmitted in accordance with the A53 standard. Revision of the A53 standard is possible and is contemplated by the ATSC, however the evolution must occur such that that existing, or so-called legacy, digital television receivers may continue to be used. Similarly, encoding and transmission of signals in accordance with the existing A53 standard may be referred to as legacy encoding and transmission.
FIG. 2 shows a block diagram of a receiver 200 that may be used to extract the source information from a received signal compliant with the existing or legacy A53 standard. An antenna 202 develops a received electrical signal from an electromagnetic signal transmitted through the airwaves. An analog-to-digital (A/D) converter 204 produces digital samples of the received signal and a trellis decoder 206 decodes the digital samples to produce a stream of trellis-decoded estimates of bits in the data stream. A/D converter 204 may also include additional front end processing circuits such as a tuner for receiving a desired channel within the received signal. In accordance with the existing or legacy A53 standard, the trellis decoder 206 includes a signal de-multiplexer, twelve ⅔-rate trellis decoders and a signal multiplexer. The de-multiplexer distributes the digital samples among the twelve ⅔-rate trellis decoders and the multiplexer multiplexes the estimates generated byte each of the twelve ⅔-rate trellis decoders.
A convolutional de-interleaver 208 de-interleaves the stream of trellis-decoded bit estimates, producing sequences or packets arranged to include 207 bytes. The packet arrangement is performed in conjunction with the determination and identification of the location of the synchronization signals, not shown. A Reed-Solomon error correction circuit 210 considers each sequence of 207 bytes produced by the de-interleaver 208 as one or more codewords and determines if any bytes in the codewords or packets were corrupted due to an error during transmission. The determination is often performed by calculating and evaluating a set of syndromes or error patterns for the codewords. If corruption is detected, the Reed-Solomon error correction circuit 210 attempts to recover the corrupted bytes using the information encoded in the parity bytes. The resulting error-corrected data stream is then de-randomized by a de-randomizer 212 and thereafter provided to a data decoder 214 that decodes the data stream in accordance with the type of content being transmitted. Typically, the combination of the trellis decoder 206, the de-interleaver 208, the Reed-Solomon decoder 210, and the de-randomizer 212 are identified as an 8-VSB decoder 220 within receiver 200. It is important to note that, in general, the typical receiver for receiving signals compliant with the legacy A53 standard performs the receiving process in the reverse order of the transmitting process.
In general, the algorithms employed in Reed-Solomon encoding and decoding are well known to those skilled in the art. As described above, the Reed-Solomon encoder 106 of FIG. 1 generates a codeword that has 207 bytes by adding 20 parity bytes to a data packet having 187 bytes. The Reed-Solomon decoder 210 of FIG. 2 uses the 20 bytes added by the encoder to correct errors in up to 10 bytes of the codeword.
The Reed-Solomon error correction algorithm takes advantage of the properties of a Galois Field. Specifically, a Galois Field GF(pn) is a mathematical set comprising a finite number of elements p″ where the values of p and n are integers. A particular Galois Field is defined using a generator polynomial g(x). Each element of the Galois Field may be represented by a unique bit pattern having n bits. Furthermore, a unique polynomial of degree pn may be associated with each element where each coefficient of the polynomial is between 0 and p−1. Further, mathematical operations in the Galois Field have important properties. Addition of two elements of the Galois Field GF(pn) is defined as an element associated with a polynomial that has coefficients that are the modulo-p sum of the coefficients of the polynomials associated with the two elements being added. Similarly, multiplication of two elements is defined as the multiplication of the polynomials associated with the two elements modulo the generator polynomial g(x) associated with the Galois Field. Addition and multiplication operators are defined over the Galois Field such that the sum and product of any two elements of the Galois Field are elements of the Galois Field. A property of the Reed-Solomon codeword is that multiplying each byte of the codeword by an element of the Galois Field results in another valid Reed-Solomon codeword. Furthermore, byte-by-byte addition of two Reed-Solomon codewords produces another Reed-Solomon codeword. The legacy A53 standard defines a 256 element Galois Field GF(28) and the associated generator polynomial g(x) for use in the Reed-Solomon algorithm. The properties of the Galois Field also create the ability to generate syndromes for the codewords in order to determine errors.
The output packets from the de-randomizer are provided to the data decoder 214. The data decoder 214 uses the PID in the header of the decoded packet to determine the type of information carried in the packet and how to decode such information. The PID in the header is compared to information in a Program Map Table (PMT) that may be periodically transmitted as part of the data stream and updated in the receiver. The data decoder 214 ignores any packet that has a PID for data packets that are not of a recognized type. In this manner, the legacy A53 standard allows for the creation of a new packet type not contemplated in the original standard by allowing a transmission source to assign a unique PID value for the new packet type. Legacy decoders that do not support the new packet type may ignore such packets while new decoders that do recognize the new packet type can process such packets.
As should be apparent, only those packets that are properly decoded by the ⅔-rate trellis decoder 206 and the Reed-Solomon decoder 210 in the receiver 200 are going to be provided to the data decoder 214. If the trellis decoder 206 or the Reed-Solomon decoder 210 cannot decode a packet, the receiver generally discards such packet as an error packet. If too many error packets are received, some receivers capable of receiving signals compliant with The A53 Standard may attempt to resynchronize with the transmitter.
It is important to note that signals compliant with the A53 standard, in general, may be transmitted in a manner other than over the air, including transmission over co-axial cable or telephone lines.
The existing or legacy A53 standard, at present, defines generating and transmitting a signal for the intended use by receivers that are generally fixed (e.g., in a home) and that are coupled to large antennas for capturing the transmitted signal. However, the transmitted signals are not sufficiently rugged or robust to allow a mobile receiver or a receiver with a small antenna that are used in portable televisions, vehicular televisions, cellular telephones, personal data assistants, etc. to effectively extract the source data encoded in such signals. In particular, the redundancy provided by the ⅔-rate trellis encoder is not sufficient and lower rate encoders (i.e., those that have greater redundancy) are necessary for mobile applications. Therefore it is desirable to introduce more robust encoding processes adapted to better perform with advanced receivers in mobile, handheld and pedestrian devices.